Patent GB-847350-A (GB)
Title: Improvements in or relating to doppler radar systems
IPC: G01S13/60
Filed: 1959-01-07
Abstract: 847,350. Radar. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. Jan. 7, 1959 [Jan. 15, 1958], No. 694/59. Class 40(7). A Doppler radar system measuring the relative velocity between the system and an extended surface, the motion being in a direction parallel to the surface, is arranged so that it is possible to estimate any error due to the surface providing a degree of specular rather than isotropic reflection. It is also possible to obtain information as to the smoothness or roughness of the surface. One application is to airborne speed and drift measuring systems in which the smoothness of the sea beneath the aircraft introduces a varying error into the velocity determinations; the error being greatest over the smoothest water. Owing to the radar aerial beam-width and the oblique angle at which the beam intersects the surface the maximum return is obtained from a point slightly closer than the point of intersection of the surface and the beam axis. Error due to this may be corrected in the velocity indications for travel over land surfaces but there will still be a further error over water varying with the smoothness of the water. To determine this error it is proposed to radiate from the craft a second comparison or test beam which is directed at the surface at a different angle and/or which has a maximum return angle different from that of signals derived from the normal beam. By a knowledge of the aerial directivities of the two beams and the deduced velocities from each it is possible to ascertain the correct velocity by consulting a set of curves prepared from actual measurements. Information as to the condition of the sea surface can also be deduced. In one embodiment the centre line of the aerial beam A (Fig. 4) is not changed but the test beam is wider than the normal beam so that the direction of maximum return is BO for the normal beam and CO for the test beam. Manual or automatic beam-width changeover may be incorporated in the radar and, with the latter, velocity error could be corrected automatically so that the indicator reads true velocity. In another embodiment (Fig. 6) the test beam is also wider than the normal beam but in this case its directivity (OD) is adjusted relative to that (OA) of the normal beam until equal velocity indications result. The extent of the angular adjustment will be a function of sea state and will, as before, provide a basis for determining the error in the velocity measurement. In this embodiment it would probably be necessary to provide separate aerials for the normal and test beams. In a third embodiment (Fig. 8) the normal and test beams are both fixed and have different angles of incidence on the surface. Two antennas, or a single antenna switched in directivity, are required and the co-operating radar apparatus would require adjustment to take into account the change in the angle of incidence at each change of beam. Alternatively two separate continuously operating radar systems may be used. Besides airborne speed and drift measurement apparatus the invention, utilizing sub-millimetre, or infra-red frequencies as necessary, may be applied to monitor the surface state of a web continuously passing a test point. Monitoring of paper-making plant can be performed in particular in accordance with the embodiment of Fig. 8; the use of two separate radar systems is preferred.